Vascular endothelial growth factor (VEGF) is a major mediator of neovascularization in embryonic development and adult vascular homeostasis. VEGF contributes to adaptive angiogenesis in ischemic tissue as well as excessive vessel formation in proliferative vascular pathologies. VEGF and its signaling pathways thus may provide a means of either promoting angiogenesis in vascular insufficiency or inhibiting angiogenesis in diabetic retinopathy or in tumors. Responses to VEGF are mediated by two endothelial cell-surface receptors, Fit-1 and KDR. An RNA processing variant of Fit-1 mRNA produces a secreted form of Fit-1 ("sFIt-1 ") that binds VEGF with high affinity and can inhibit biological responses to VEGF. Previous studies indicate that the ratio of sFIt-l:FIt-1 can vary under physiological conditions, suggesting that molecular mechanisms exist to specifically control the mRNA processing events leading to sFIt-l. We postulate that the relative expression of sFIt-1 and Fit-1 is a significant biological determinant of vascular responsiveness to VEGF. The goals of the proposed research are to identify new experimental systems and biochemical assays to discover the regulatory mechanisms governing sFIt-1 expression. Our specific aims are 1) to identify biological models appropriate for the study of regulated sFIt-1 expression, 2) to identify nuclear factors involved in Fit-1 pre-mRNA processing, and 3) to determine the post-transcriptional mechanism by which protein kinase C activation alters sFIt-1 expression. In Aim 1, we will quantify sFIt-1:Fit-1 expression in cell- or animal-based models of vascular development and adult angiogenesis. Under Aim 2, we will probe the interaction of specific and unknown processing factors with Fit-1 pre-mRNA using cell-free systems. In Aim 3, we will assess the role of altered mRNA stability and pre-mRNA 3rocessing in the induction of sFIt-1 expression by phorbol ester. These studies will increase our understanding of the mechanisms of both adaptive and therapeutic angiogenesis, and may provide rationale for developing novel pharmacological interventions to modulate sFlt-1 expression and, thereby, VEGF responsiveness.